Photodynamic Disinfection (PDD) has been demonstrated to be an effective non-antibiotic antimicrobial approach in vitro. One exemplary advantage of PDD as an antimicrobial treatment modality is that, due to its non-specific bactericidal mechanism, it is typically not subject to issues of resistance that can plague the use of antibiotics. As another exemplary advantage, it can be employed as a localized topical treatment that can be administered in areas such as the oral or nasal cavities where sustained topical antibiotic delivery can be problematic. For these reasons and others, PDD is fast becoming a valuable tool in the treatment of bacterial-related conditions such as periodontal disease.
PDD fundamentally involves the use of light energy to activate one or more photosensitizers of a photosensitizer composition so that those photosensitizers can then either interact directly with a substrate/target (type I reaction), or can interact with molecular oxygen to produce singlet oxygen and other reactive oxygen species (type II reaction). These reactions mediate the non-specific killing of microbial cells primarily via lipid peroxidation, membrane damage, and damage to intracellular components. In order for this process to kill and/or reduce microbes, it is typically desirable that the photosensitizer be internalized into or brought into very close association with the cellular envelope of such microbes.
The esters of p-hydroxybenzoic acid, commonly referred to as parabens, are among the most commonly used preservatives in cosmetic and pharmaceutical formulations. They offer the advantages of broad antimicrobial activity, efficacy over a wide pH range, low toxicity, and low sensitization potential. In addition, these compounds are relatively odorless, colorless and highly stable. Parabens are typically most effective against fungi and Gram positive bacteria. Combinations of parabens such as methylparaben (methyl-4-hydroxybenzoate) and propylparaben (propyl-4-hydroxybenzoate) offer greater preservative activity and improved solubility over individual parabens, and these combinations are commonly used in commercial products.
The mechanisms of antimicrobial action of parabens are just beginning to be fully understood, and evidence for several processes has been proposed. Furr and Russell detected intracellular leakage of RNA when Serratia marcescens was exposed to parabens indicating disruption of cellular membrane transport processes (see J. R. Furr and A. D. Russell, Factors influencing the activity of esters of p-hydroxybenzoic acid on Serratia marcescens, Microbios, 1972). Freese, et al. determined that parabens interfered with both membrane transport and electron transport systems (see E. Freese, C. W. Sheu and E. Galliers, Function of lipophilic acids as antimicrobial food additives, Nature, 1973, 241:321-325).
Eklund found that parabens eliminated the change in pH of the cytoplasmic membrane, but did not disrupt the membrane potential component of the proton motive force. He subsequently concluded that neutralization of the proton motive force and subsequent transport inhibition could not be the only mechanism of inhibition (see T. J. Eklund, The effect of sorbic acid and esters of p-hydroxybenzoic acid on the protonmotive force in Escherichia coli membrane vesicles, Gen Microbiol 1985, 131:73-6).
Work by Panicker with dipalmitoyl phosphatidic acid vesicles, a membrane system model, revealed a concentration dependent interaction of propylparaben with the lipid membrane. At low concentrations propylparaben altered membrane function by interacting with the cell wall allowing passive transmembrane diffusion to the target receptor. At high concentrations, propylparaben interacted with lipid components making the cell wall more rigid. This in turn altered membrane semipermeability and thus membrane function (see L. Panicker, Effect of propyl paraben on the dipalmitoyl phosphatidic acid vesicles, J Colloid Interface Science, 2007, 311:407-416).
Bredin, et al. reported that membrane destabilization was induced when E. coli was exposed to propylparaben. Upon exposure, potassium was released in a manner similar to polymyxin B induction of outer membrane permeabilization. Furthermore, this efflux was dependent upon porin channel activity (see J. Bredin, A. Davin-Regli and J. M. Pages, Propyl paraben induces potassium efflux in Escherichia coli, J Antimicrobial Chemo. 2005, 55:1013-1015).
Nguyen, et al. showed that ethyl- and propylparaben interacted with the mechanosensitive channels of large and small conductance thereby disrupting osmotic gradients in E. coli (see T. Nguyen, B. Clare, W. Guo and B. Martinac, The effects of parabens on the mechanosensitive channels of E. coli, Eur Biophys J., 2005, 34(5):389-95).
Finally, in addition to membrane disruptive mechanisms, Ma, et al. showed that parabens were highly effective inhibitors of bacterial enzyme systems. For example, F-ATPases were reversibly inhibited in Streptococcus mutans, but the membrane protein (Enzyme II) of the phosphopyruvate: phosphotransferase system for glucose uptake and phosphorylation was irreversible inhibited (see Y. Ma, and R. E. Marquis, Irreversible paraben inhibition of glycolysis by Streptococcus mutans GS-5, Lett Appl Microbiol, 1996, 23:329-33).